1. Field of the Invention
The present invention relates to a portable telephone apparatus for performing wireless communication through a base station in a first portable telephone system that uses a predetermined communication protocol, and to the improvement of roaming between the first portable telephone system and a second portable telephone system that uses a different communication protocol.
2. Description of the Background Art
In recent years mobile phone infrastructure has been developed in industrialized nations such as Japan, the US and European countries, as well as various other countries around the world.
Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and Code Division Multiple Access (CDMA) are examples of systems generally used in wireless communication. CDMA is thought to have particular potential as it has superior multipath phasing measures and allows better communication in built up areas.
The CDMA mobile phone system was developed by the US company Qualcomm. After being adopted as US standard IS-95 in 1993, the system came into mainstream use and is now employed in numerous countries including Hong Kong (since September 1995) and South Korea (since January 1996). Meanwhile, the IS-95 standard was revised to IS-95A and then to IS-95B. Japan employs a mobile phone system ARIB STD-T53 (commonly known as T53) that conforms with IS-95A.
As mobile phone system infrastructure is improved in various countries, the appearance of portable telephones that are capable of international roaming is anticipated.
A portable telephone apparatus capable of international roaming can be used in a first country, and, when taken from the first country to a second country, can also be operated in the second country. Here, it is possible that a portable telephone apparatus compatible with the US IS-95 system could also be used in Hong Kong and South Korea, and thus so-called international roaming could be performed between the US and Hong Kong and the US and South Korea. This is because Hong Kong and South Korea use the same IS-95 system as the US, meaning that providing a portable telephone apparatus performs procedures compatible with IS-95, communication can take place with the mobile phone system in any of these countries.
However, when a portable telephone that can be used in the US IS-95 protocol mobile phone system is taken to Japan, which uses the ARIB STD-T53 communication protocol mobile phone system, the portable telephone apparatus performs procedures in compliance with IS-95, but the system performs procedures in compliance with T53. Here the procedures performed by the portable telephone do not match those performed by the system. As a result the portable telephone apparatus taken to Japan cannot perform communication, making international roaming between Japan and the US impossible.
In a portable telephone apparatus, the procedures performed according to the communication protocol are implemented in the program code. Therefore if program code for procedures in both IS-95 and T53 is packaged in the apparatus in advance, international roaming between countries that use different protocols, such as between Japan and the United States, should be possible. However, if program code for procedures in both IS-95 and T53 is packaged in a portable telephone apparatus, the program code size will be double that of when program code for only one protocol is packaged. If the code size is doubled it is necessary to double the size of the memory that stores the code, resulting in high manufacturing costs for such a portable telephone apparatus.